Drs. Steffanie A. Strathdee, María Remedios Lozada Romero, and Carlos Magis-Rodríguez are working to understand HIV risk behaviors among drug users and sex workers on the Mexico-U.S. border, identifying key factors in the rapidly increasing risk of HIV infection in Tijuana that could lead to a resurgence throughout Mexico and Central America. An outstanding example of cross-border cooperation, the trio has worked effectively to document factors—such as injecting drug use, commercial sex work, geographic mobility, U.S. deportation patterns, history of other sexually transmitted infections, and gender—that significantly impact the evolving HIV epidemic in Tijuana. Their public health approach to shared responsibility for recognizing and treating sexually transmitted diseases between bordering countries contributes significantly to improved cross-border HIV prevention, clinical practice, and multidisciplinary global health training programs.